The invention relates to magnetic resonance (MR) imaging, and more particularly relates to magnetic resonance angiography (MRA) of the coronary arteries. In its most immediate sense, the invention relates to three-dimensional MRA of the coronary arteries.
Radiologists have long sought to conduct three-dimensional MRA studies on human coronary arteries. However, such efforts have not been successful. To be diagnostically useful, the images must be free of motion artifacts, must have sufficient contrast between tissue and blood, and must have a minimum level of spatial resolution. And, to be practical, the MRA study must not take too long.
It has heretofore been impossible to produce three-dimensional cardiac MRA images that are not only diagnostically useful, but practical as well. This is because cardiac motion creates motion artifacts in the MRA image. If the study is carried out using techniques that acquire MR image data sufficiently quickly that such artifacts are absent, the study becomes too lengthy. If the study is shortened, spatial resolution and tissue contrast get traded off against the presence of motion artifacts.
One object of the invention is to provide a method for imaging a patient's coronary arteries in such a manner that motion artifacts are minimized while permitting the study to be completed in a practical time.
Another object of the invention is to provide such a method in which adequate spatial resolution and tissue contrast need not be traded off against the presence of motion artifacts in the image.
Another object is, in general, to improve on methods of this general type.
The invention proceeds from the realization that the coronary arteries do not all move at the same time and that there is a time during the cardiac cycle when each coronary artery is relatively motionless. Hence, if MR data could be acquired from each coronary artery at a time when the artery of interest was essentially motionless, motion artifacts would (if not eliminated) be at least greatly reduced.
The invention also proceeds from the realization that it is possible to segment the volume of interest so as to correspond to the stationary positions of the coronary arteries of interest. In this way, the MR data collection can be restricted to regions where the data are important. Because of this, the study can be conducted more rapidly.
In accordance with the invention, a volume of interest (VOI) is established. The VOI includes all the coronary arteries to be imaged. Then, the VOI is divided into a plurality of segments. Each segment relates to a part of a coronary artery that remains relatively motionless during a corresponding portion of the patient's cardiac cycle. Thereafter, magnetic resonance (MR) data are acquired for each of the plurality of segments. For each segment, this data acquisition takes place during the corresponding portion of the patient's cardiac cycle.
Hence, in accordance with the invention, little motion is occurring while MR data are being acquired. Therefore, motion artifacts are largely absent from the reconstructed image. Furthermore, because data collection is limited only to particular segments of the VOI, the study can be conducted rapidly.
In accordance with the preferred embodiment, MR data are acquired from each one of the segments during a single cardiac cycle. During each successive cardiac cycle, more MR data are acquired from each one of the segments. This process is continued until sufficient data have been acquired.
Advantageously, and in further accordance with the preferred embodiment, the entire MR data acquisition takes place during a single breath hold. This minimizes body motion during the acquisition, which produces an image of improved diagnostic quality. Alternatively, respiratory gating methods can be used to reduce motion artifacts, including the use of navigator techniques that monitor cardiac or diaphragm motion.
Further advantageously, the MR data acquisitions are carried out using an MR pulse sequence of a type in which blood is rendered bright in a reconstructed MR image. Alternatively, or additionally, it is possible to administer a paramagnetic contrast agent to the patient before conducting the MRA study, thereby increasing the intensity of the blood signal.